As long as you're not behind on the loans when the deferment goes into effect, I don't think it has any impact at all. I've had other things impacting my scores when I've done a deferral, so I can't say for sure about the actual score. However, it does not show a reason for the deferral, so you won't get dinged for "economic hardship". And now, well after the deferral (one year plus), it does not show a gap in payment--it shows "OK" for all of the months that I was in deferral.
You can also check with the student loan company and ask them how they report it. I was dealing with Sallie Mae.